1. Field of the Invention
The present invention relates to a gas-discharge lamp controller, and more particularly to a gas-discharge lamp controller capable of adaptively generating a preheating frequency.
2. Description of the Related Art
In supplying power to gas-discharge lamps, electronic ballasts are widely adopted to keep the lamp current stable.
To increase the lifetime of gas-discharge lamps, the lighting process of the electronic ballasts should start with a preheating phase to pre heat the lamps, enter an ignition phase after the preheating phase to ignite the lamps, and then settle to a steady phase. Of the three phases, the preheating phase is required to have a preheating frequency, which varies with the characteristics of the lamps, to facilitate the ignition of the lamps and thereby prolong the lamps' lifetime.
A prior art solution for generating the preheating frequency utilizes a fixed DC voltage in the preheating phase as a control voltage for an oscillator which generates an oscillation frequency corresponding to the control voltage. Please refer to FIG. 1, which shows a block diagram of part of a ballast circuit, including a prior art gas-discharge lamp controller and some external passive components. As can be seen in FIG. 1, the gas-discharge lamp controller 100, coupled with a capacitor 105 and a series connection of resistor 106 and capacitor 107, includes an oscillator 101, a switch 102, a comparator 103, and a current source 104.
The oscillator 101, usually implemented with an astable vibrator, is used to generate a saw-tooth signal VSAW of which the oscillation period is determined by a control voltage VC and the RC time constant of the resistor 106 and capacitor 107. The lower/higher the voltage of the control voltage VC is, the shorter/longer the oscillation period of the saw-tooth signal VSAW will be.
The switch 102 has a control input end coupled to a preheating time end signal VPHE, two input ends coupled to a first voltage VH1 and a second voltage VH2 respectively, and an output end for providing the control voltage VC. When the preheating time end signal VPHE is at a low level, VC=VH1; when the preheating time end signal VPHE is at a high level, VC=VH2. The voltage of the first voltage VH1 is set to a value to make the oscillator 101 generate a desired preheating frequency of the preheating phase. The second voltage VH2 is used for the ignition phase and the steady phase.
The comparator 103, the current source 104, and the capacitor 105 are used to generate the preheating time end signal VPHE, wherein the current source 104 has a small current and is used to charge the capacitor 105 to generate a slowly increasing voltage VPHT. The comparator 103 is used to compare the slowly increasing voltage VPHT with a reference voltage VREF to generate the preheating time end signal VPHE. As the slowly increasing voltage VPHT reaches the reference voltage VREF, the preheating time end signal VPHE will change state from low to high to indicate the end of the preheating phase.
As such, each gas-discharge lamp controller 100 can generate a specific preheating frequency corresponding to a specific value of the first voltage VH1. However, if more than one preheating frequency, for example four different preheating frequencies is needed, then four different models of gas-discharge lamp controllers—corresponding to four different values of the first voltage VH1—will have to be prepared. This can cause inconvenience in manufacturing process and products management as well.
One prior art solution to this problem is to add an extra pin for generating the first voltage VH1. However, this will increase the chip size and the board area, and therefore the cost.
In view of the disadvantages of the prior art design, the present invention proposes a novel topology of a gas-discharge lamp controller, capable of adaptively generating a preheating frequency with no extra pin added.